RE: 5-40 Small Pattern Hex Nuts
You might save yourself a lot of wasted time and frustration if you simply purchased a 5-40 tap and some 3/16" brass hex and got on with it. Drill and tap as deep as you can get with the tap then get someone with a lathe to part off sections to whatever nut thickness you deem appropriate for the job.
ChrisM
'ffkiwi'